Multiple message-header fields with the same field-name MAY be
present in a message if and only if the entire field-value for that
header field is defined as a comma-separated list [i.e., #(values)].
It MUST be possible to combine the multiple header fields into one
"field-name: field-value" pair, without changing the semantics of the
message, by appending each subsequent field-value to the first, each
separated by a comma. The order in which header fields with the same
field-name are received is therefore significant to the
interpretation of the combined field value, and thus a proxy MUST NOT
change the order of these field values when a message is forwarded.

Multiple header fields with the same field name MUST NOT be sent in a
message unless the entire field value for that header field is
defined as a comma-separated list [i.e., #(values)]. Multiple header
fields with the same field name can be combined into one "field-name:
field-value" pair, without changing the semantics of the message, by
appending each subsequent field value to the combined field value in
order, separated by a comma. The order in which header fields with
the same field name are received is therefore significant to the
interpretation of the combined field value; a proxy MUST NOT change
the order of these field values when forwarding a message.

OK there is nothing to fix then!

And my server implementation is wrong as it should send a list of csv for the header Link:.
Cool!

Now what could be practical for requests is a way to access for headers which have csv list a sub-dict. In the meantime I will iterate on the list.

And my server implementation is wrong as it should send a list of csv for the header Link:.

I think you are wrong here. Firstly, there seems to be no change in rules regarding multiple header fields with the same field name between RFC 2616 and the draft of httpbis you cited. The wording has changed but the sense is the same. Secondly, RFC 5988 section 5 defines the value of Link header as a list of comma separated values (Link = "Link" ":" #link-value) which means, according to the rules you cited, that there CAN be multiple Link headers and that they CAN be merged into one, comma separated list of values.

unless the entire field value for that header field is defined as a comma-separated list [i.e., #(values)].

When the value is of a type a, b, c, d, we can't combine it in one header field because the comma for separation the values of the field, and the comma for separation of the values of the value of the field would not be parseable anymore in the right components.

To be clearer, let's imagine a field name Foo: a, b, c, d defined as it is. The value is a list of 3 or 4 tokens.

Foo: a, b, c, d
Foo: e, f, g

You can NOT combine these two headers because it would not be possible to parse.

Note: The "Set-Cookie" header field as implemented in practice can occur multiple times, but does not use the list syntax, and thus cannot be combined into a single line ([RFC6265]). (See Appendix A.2.3 of [Kri2001] for details.) Also note that the Set-Cookie2 header field specified in [RFC2965] does not share this problem.

Well, it's interesting how seemingly simple things are difficult to specify so that there's no room for different interpretations :)

(...) unless the entire field value for that header field is defined as a comma-separated list [i.e., #(values)]

I think the above statement is not entirely clear by itself. I guess the assumption is, no value (from values) includes unquoted comma. If so then taking your example it would be legal to combine

Foo: a, b, c, d
Foo: e, f, g

into

Foo: a, b, c, d, e, f, g

as long as none of the letters represents value with unquoted comma. The reasoning being, all values can be parsed and all are treated equally as long as order is intact.

And the other hand, Link can be combined in one header.

In two places RFC 5988 mandates quoting values if they contain semicolon or comma, indeed. We would have to follow whole grammar and make sure all possible commas are quoted but I guess this is the case thus multiple Link header fields can be combined into one. However, I wasn't arguing with this (on the contrary, I stated the same myself) but with your statement And my server implementation is wrong as it should send a list of csv for the header Link:. which I think is not true.

@sigmavirus24 because it is still not sure sure if it has be reopened or not. Trying to understand the issue first. In the other hand, if there is a better place for it. I will be happy to discuss it there.